272 Produce Degradation: Reaction Pathways and their Prevention
The movement of liquids across plant tissues is limited by the plasma membrane,
which acts as a barrier. The plasma membrane is selectively permeable and allows
the movement of water and other small uncharged particles across the membrane
rather than large charged solutes. In osmosis, both the movements of substances
down a concentration gradient and down a pressure gradient have an influence on
the mobility of water as a result of increased solute dissolution. The mechanism of
osmosis involves diffusion of single water molecules across the membrane bilayer
and bulk flow through tiny water-filled pores of molecular dimensions; the driving
force is the water/chemical potential gradient (Devlin and Witham, 1983).
9.4 WATER MIGRATION IN PLANT TISSUES
Water in liquid phase travels across membranes, through the cell’s hollow channels
and cell walls, and escapes as water vapor into the air spaces inside the leaf and
diffuses to the environment. The gradient in water vapor concentration plays a
significant role in the transpirational water loss from the leaf surfaces. Long-distance
water transportation in the xylem is dependent on a pressure gradient, while that
through cell layers responds to water potential gradients.
Water transportation from the roots to the leaves occurs through the apoplast
and symplast routes. The symplast is an extensive interconnection of cytoplasmic
cells that form a continuous system and hold as much as 70% of the water in plant
tissues. Water in the symplast includes water within the plasma membrane and in
the cytoplasm and the central vacuoles; 80 to 90% of the symplast water is contained
in the vacuole. Water in the symplast travels from one cell to the next through the
plasmodesmata as a result of the energy gradient. External to the symplast is the
apoplast, which is a continous system of cell walls and intercellular air spaces in
plant tissues through which water and solutes can also traverse. In the apoplast,
water mainly travels through the cell wall without crossing any membranes. The
plasma membrane of the cell may restrict movement of water and solutes to the
apoplast. Once water is in the apoplast, mobility is toward areas of lower chemical
potential. Both the symplast and the apoplast provide routes for movement of water
and solutes. Water is mainly transported through the apoplast; solutes not able to
diffuse across membranes are transported through the symplast. The transmembrane
pathway is another route that carries water through the plant system and involves
movement of water from one cell to another; water crosses at least two membranes for
each cell in its path (Devlin and Witham, 1983; Steinbeck, 1995; Taiz and Zeiger, 1998).
The great majority of water transportation from the roots to the leaves occurs
through the xylem, a simple pathway offering low resistance to water movement.
The negative hydrostatic pressure created by leaf transpiration pulls water up the
water column in the xylem (Kramer, 1995). Water movement in the xylem occurs
through two types of specialized tracheary elements, the tracheids and the vessel
elements. The tracheids communicate with adjacent tracheids through numerous
microscopic pits, which are regions where the secondary wall is absent and the
primary wall is thin and porous. The pits offer low resistance for water movement
between the tracheids. Vessel elements have perforated end walls that form a per-
foration plate at each end of the cell and also contain pits in their walls. Vessels